Question

Does something have to be more or less dense to float?

Answer 1

it has to be less dense to float, ex. water and PE plastics where water is density 1000 kg/m3 and PE plastics is 900-950 kg/m3 depending what type of PE plastics it is...

Answer 2

yep, or take water and ice, ice is less dense (because the static network of hydrogen bonding the molecules make and, averaged out, each molecule takes up more space) than water (molecules can be more compressed and take up less space), so it floats.

density = mass / volume